The distribution and abundance of fire ant decapitating flies (Diptera: Phoridae: Pseudacteon Coquillett) were studied in three regions of southern South America, primarily from September 2002 to September 2004. A total of 2,421 flies belonging to 14 Pseudacteon species were found at 51% of the 662 fire ant mounds examined at 125 collecting sites. Flies occurred in a variety of habitats at altitudes from sea level to 2,280 m. Pseudacteon obtusus Borgmeier (large form) was found at the highest altitude and at the most western longitude. Flies were active between 16 and 37°C, 20 and 90% RH, and 0 and 11.6 km/h wind speed. Pseudacteon curvatus Borgmeier showed the highest abundance and one of the broadest geographical distributions. Pseudacteon tricuspis Borgmeier, P. litoralis Borgmeier, the large form of P. obtusus, P. nudicornis Borgmeier, and P. nocens Borgmeier also were widely distributed. These species seem to be the most generalized within saevissima-group. Pseudacteon solenopsidis Schmitz was only collected attacking isolated workers. A new Pseudacteon species was discovered in northwestern Argentina. Seven fly species were reported for the first time on a new fire ant host in this region. Pseudacteon cultellatus Borgmeier was found for the first time on Solenopsis invicta Buren in Corrientes province in northeastern Argentina, where up to nine fly species have been found to cooccur. Males of P. tricuspis and P. obtusus were the only males normally attracted to disturbed fire ant colonies.